Copper foil used in printed circuit boards is thermally pressed on a resin board. A surface of the thermally pressed copper foil may or may not be roughened. The roughened surface of the copper foil may increase bonding strength between the resin board and the copper foil. If the surface of the copper foil is not roughened, the bonding strength may be increased using an adhesive or a silane coupling agent. As a result, the printed circuit board may have high reliability.
As electronic components are highly integrated, miniaturized, and made lightweight, wiring patterns of printed circuit boards become finer. When a thick copper foil is used to prepare a printed circuit board having fine patterns, the time taken to etch the printed circuit board for forming the wiring patterns is increased. Thus, verticality of sidewalls of the wiring patterns formed by etching may be reduced. In particular, if a wiring pattern formed by etching has a narrow line width, the wiring pattern may be discontinuous. Thus, the thickness of the copper foil used for fine patterns may be equal to or less than 9 μm. In general, a copper foil having a thickness of 12 μm is thermally pressed and half etched to be used for fine patterns. In order to reduce the loss of copper foil in a process such as etching, the copper foil may be formed to be ultra thin. The thickness of the ultra-thin copper foil, which is not attached to a carrier, is about 9 μm.
However, since the ultra-thinned copper foil (hereinafter, ultra-thin copper foil layer) has weak mechanical strength, it may be wrinkled or broken during the preparation of a printed circuit board. In addition, the ultra-thin copper foil may be cut. Accordingly, an ultra-thin copper foil to which a carrier foil is attached may be used for micro patterning. The ultra-thin copper foil to which a carrier foil is attached may be prepared by electrically depositing an ultra-thin copper foil on a surface of a metal foil functioning as a carrier (hereinafter, carrier foil) using a peeling layer.
That is, in the ultra-thin copper foil to which a carrier foil is attached, the peeling layer is formed on a surface of the carrier foil and the ultra-thin copper foil is formed thereon using electroplating. In general, the outermost surface of the ultra-thin copper foil formed by electroplating is roughened.
The peeling layer formed on a surface of the carrier foil is formed of an organic material, Cr, a Cr alloy, a Ni alloy, or the like. The peeling layer is disclosed in Japanese Patent Publication Nos. 2002-368365, 2004-031375, 2004-169181, 2004-161840, 2005-254673, 2005-260058, 2006-022406, and 2006-312265.
The peeling layer may be a metal layer formed by metal plating or an oxide film which is formed by metal plating followed by anodic oxidation, and then it is peeled off from the ultra thin copper foil. The peeling layer has good peelability at room temperature but non-uniform peelability at high temperature.
In a printed circuit board, a high temperature resistant resin, such as, polyimide, is used as an insulating board. Thus, pressing processes between the ultra-thin copper foil and an insulating board and/or curing processes of the printed circuit board should be performed at high temperature, and an organic peeling layer attached to the ultra-thin copper foil is not easily peeled off. Thus, a metallic peeling layer instead of an organic peeling layer is used. In this regard, the metallic peeling layer may be formed of Cr, a Cr alloy, or the like. For example, Korean Patent Publication No. 2005-0025277 discloses a method of forming a peeling layer using Cr. However, the use of Cr is limited according to the Restriction of Hazardous Substances Directive (Rohs), and stability of printed circuit boards using such metallic peeling layers is reduced due to blisters occurring during the preparation process of the printed circuit boards and a variation of peelability.
Thus, there is a need to develop a peeling layer without using Cr, wherein, when the peeling layer is used in the preparation of printed circuit boards, blister does not occur.